AC Voltage Application Notes
Many requests are received regarding the permissible AC voltage/frequency capability of metallized polyester film capacitors, because they are smaller and appear to have a temperature advantage when compared with metallized polypropylene capacitors of similar capacitance and voltage rating. It has also been asked why we do not publish these curves in our catalog as do some other capacitor suppliers. The information below will help to address this question. This information is not presented as a detailed analysis, but rather to highlight some of the issues that need to be considered when using metallized polyester film capacitors in AC applications.
There is no such thing as a “perfect” film capacitor dielectric. All have their unique “pros” and “cons”. As an example, polypropylene film has many advantages: it behaves very uniformly and predictably over temperature and frequency allowing suggested operating limits to be calculated and plotted. It has very low dielectric losses. Its voltage withstand per unit thickness is the highest of all capacitor films. However, polypropylene has a maximum temperature limit of +105°C. It also has a low dielectric constant, which results in a larger physical size for a given capacitance and voltage rating when compared with most other film dielectrics. Therefore the reduced size and perceived temperature advantage of metallized polyester film capacitors are strong motivators to consider their use.
For metallized polypropylene film capacitors the increase in dissipation factor associated with increasing frequency is determined by the ohmic losses of the lead wires and the metallization alloy deposited on the film. The dielectric losses are very small and do not change. This behavior lends itself to spreadsheet analysis to create performance curves that conservatively represent real behavior in applications. Defining the capacitor application thermal environment enables scaling these performance curves as required.
Defining the voltage/frequency capability of polyester film capacitors is particularly more complicated because:
- Polyester film dielectric losses are usually more than an order
of magnitude higher than for polypropylene film, and
- Polyester film losses are a very strong function of frequency and temperature.
These film losses are high enough to completely dominate ohmic losses in polyester capacitors. More important, the DF behavior with temperature and with frequency are transcendental functions that cannot be modeled easily with a spreadsheet.
If you refer to the Typical Temperature Characteristics graph you will find a Dissipation Factor vs. Temperature graph for polyester film at 1KHz. Note that as you move above +50°C the DF starts to climb. For large enough AC voltages the increasing losses with temperature can result in thermal runaway. It can also be seen that for AC applications the +125°C internal hot spot temperature limit is extremely misleading. It is very difficult to use the temperature range between +85°C and +125°C for AC applications because the allowable AC voltage drops so fast with increasing temperature!
The DF vs. temperature behavior changes drastically with frequency. Upon request we can provide graphs of DF vs. temperature for several frequencies, and describe an iterative method for providing a “starting point” for polyester capacitor performance evaluation in an AC application. It will be left in the hands of the product design engineer to do the temperature rise tests under worst case electrical and thermal conditions to verify that a proposed polyester capacitor is indeed suitable for that specific application.
Since a very small ambient temperature change can result in a large increase in DF, the polyester capacitor behavior can be VERY sensitive to the application thermal environment. That environment can be so variable that in our opinion publishing generally applicable performance curves would be very misleading!
However we also realize that despite the information presented above there are many AC applications where polyester capacitors represent the best solution from a size, cost, and electrical performance perspective.
We hope you find this information useful in your design process, and we welcome technical discussions on the suitability of any type of film capacitor for your application.